1. Field of the Invention
The present invention relates to a surface-coated cermet cutting tool (hereinafter, referred to as a coated cermet tool) of which a hard coating layer exhibits excellent chipping resistance, in particular, during high-speed intermittent cutting of steel, cast iron, etc.
2. Description of the Related Art
Conventionally, a coated cermet tool is known, which is generally formed by coating, on a surface of a substrate (hereinafter, generally referred to as a tool substrate) made of tungsten carbide (hereinafter, referred to as WC)-based cemented carbide or titanium carbonitride (hereinafter, referred to as TiCN)-based cermet, a hard-coating layer including the following upper and lower layers (a) and (b):
(a) as the lower layer, a titanium compound layer having at least one or two of titanium carbide (hereinafter, referred to as TiC) layer, a titanium nitride (hereinafter, referred to as TiN) layer, a titanium carbonitride (hereinafter, referred to as TiCN) layer, a titanium carboxide (hereinafter, referred to as TiCO) layer, and a titanium oxycarbonitride (hereinafter, referred to as TiCNO) layer, all of which are formed by chemical vapor deposition, the titanium compound layer having a total average thickness of 3 to 20 μm, and
(b) as the upper layer, a deposited α-type aluminum oxide (hereinafter, referred to as Al2O3) layer having an α-type crystal structure deposited by chemical vapor deposition and an average thickness of 1 to 15 μm. It is also known that the coated cermet tool is widely used for, for example, continuous or intermittent cutting of steel or cast iron.
Generally, it is also well known that a titanium compound layer or the Al2O3 layer constituting the hard-coating layer of a coated cermet tool has a granular crystal structure, and further the TiCN layer constituting the titanium compound layer has a longitudinal growth crystal structure formed by carrying out chemical vapor deposition in a moderate temperature range of 700 to 950° C. using as a reaction gas a mixed gas which includes organic carbonitride, for example, CH3CN in a conventional chemical vapor deposition reactor for the purpose of increasing the strength of the layer.
[Patent Document 1] Japanese Unexamined Patent Application Publications No. 6-31503
[Patent Document 2] Japanese Unexamined Patent Application Publications No. 6-8010.
In recent years, the performance of cutting tools has been markedly enhanced, and demands for labor saving and energy saving in cutting work and cost reduction have been increased. Accordingly, the cutting work is more often carried out at a higher speed range. The conventional coated cermet tools generally present no problem when they are used in the continuous cutting or intermittent cutting of steel, cast iron or the like under normal conditions. However, when the conventional cutting tools are used in a high-speed intermittent cutting under the severest cutting condition, i.e., in the high-speed intermittent cutting where mechanical and thermal impacts are repeatedly applied to the cutting edge at very short pitches, a titanium compound layer which is typically the lower layer of a hard-coating layer has high strength and exhibits excellent impact resistance. However, the deposited α-type Al2O3 layer that constitutes the upper layer of a hard-coating layer, despite its hardness in high temperature and excellent heat resistance, is very brittle against the mechanical and thermal impacts. As a result, chipping easily occurs in the hard coating layer, consequently shortening the usable life of cermet cutting tools.